1. Field of the Invention
This invention generally relates to a method and apparatus for combating co-channel NTSC interference for digital TV transmission, and more particularly, to the use of a variable-comb filter in combating co-channel NTSC interference for digital TV transmission.
2. Discussion of the Related Art
The Federal Communications Commission (FCC) and cable television testing organizations, such as CableLabs, have been evaluating digital television delivery systems in order to choose a new television "standard" which someday will replace NTSC in the United States. These systems all involve digital coding and data compression techniques, for example those utilizing Motion Picture Experts Group (MPEG) algorithms or variations thereof.
The FCC plans to test and approve an advanced television (ATV) standard for terrestrial broadcasting comprising, for example, high definition television (HDTV) and standard definition (SDTV) digital signals for terrestrial broadcasting. Although the specifics of the standard are yet to be fully tested and agreed upon, the FCC has indicated that the system will initially take the form of a so called "simulcast" approach. The new HDTV signals will have to fit into currently unused television channels (so-called "taboo" channels) and initially co-exist with conventional analog television signals without co-channel interference.
NTSC will be used hereinafter to represent one example of conventional television broadcasting. Other examples would be SECAM and PAL. Although NTSC is exemplified herein, it is not meant to be construed as a limitation and will be used herein synonymously with "conventional" to represent conventional television in general.
In 1994, the FCC tested a so-called "Grand Alliance" system, a system which is being developed cooperatively by corporate sponsors thereof who were involved in a first round of individual proposals tested by the FCC in 1991 and 1992. This newly developed system proposes to take the best features from those systems already tested. This is being done in order to present a single optimum system for FCC approval as the U.S. standard.
The Grand Alliance has decided on a coding algorithm which will comply with the source coding standards proposed by the MPEG. In addition, an RF transmission approach developed by Grand Alliance member Zenith Electronics Corporation was selected by the Grand Alliance. The RF transmission approach utilizes multi-level vestigial sideband (VSB) modulation which is described in "VSB Transmission System Grand Alliance: Technical Details", Feb. 18, 1994.
In the Grand Alliance system, a comb-filtering approach is used to alleviate the degradation in performance of the "simulcast" HDTV transmission system caused by a co-channel conventional television transmission. This comb-filtering approach is based on an appropriate choice of symbol-rate and center frequency of a digital TV signal, which then allows the use of a comb-filter with a delay element of 12 symbol intervals to effectively remove the visual, sound, as well as the chrominance carders corresponding to the interfering co-channel NTSC spectrum (as described in "VSB Transmission System Grand Alliance: Technical Details").
However, when a co-channel NTSC spectrum is absent, the use of a post-comb filter at the HDTV receiver causes an undesirable loss in error performance of 3 dB when only Additive Wide Gausian Noise (AWGN) is present. This loss is due to the structure of the comb filter. Such a comb filter has a single delay of 12 symbols adding to a direct symbol path thus causing noise to be added as well. Such a system is discussed in U.S. Pat. No. 5,086,340, incorporated herein by reference.
A trellis encoder is used in conjunction with 8 VSB modulation in the Grand Alliance system. The use of the trellis encoder in conjunction with the 8 VSB modulation actually causes a loss of 3.5 dB in noise performance when used in combination with a comb filter, as discussed in "VSB Transmission System Grand Alliance: Technical Details". Hence, in the Grand Alliance system, an alternate path is provided at the receiver for a case when co-channel NTSC is absent. Such an alternate path eliminates the use of the comb filter at the receiver via a switching action. A decision on whether to use the comb filter at the receiver or not is determined by a measured error-rate of periodically sent data field sync symbols at the outputs of a circuit with the comb filter and a circuit without the comb filter, respectively. Whichever measured error is smallest at the end of a preset period determines whether or not the comb filter is used. Such a switching action between a comb filter circuit and a no-comb filter circuit is cumbersome and unreliable.
Another problem with this comb-filtering approach is that when both co-channel interference and AWGN are present, a performance of the comb filter degrades dramatically. This is because the AWGN, after the comb filter, does not remain white, but gets "colored". This coloring of the AWGN affects the performance of a trellis decoder used in the HDTV receiver, noting that the trellis decoder is optimized for performance in an AWGN channel. Since a co-channel conventional television interference is maximum at a fringe area where a signal power is small and hence a noise component is large, this is indeed a scenario which must be taken into account.
One method to avoid degradation in performance when both co-channel NTSC and AWGN are present is to use a variable comb filter at the receiver as discussed in U.S. Pat. Nos. 5,162,900 and 5,087,975, incorporated herein by reference. Unfortunately, the variable comb filter arrangements of the '900 and '975 patents are subject to error-propagation. That is, a single decision error can cause multiple decision errors, and hence, cause a degradation in performance which may, in certain cases, be more than the degradation caused by using only a comb filter.
Another problem associated with using a variable comb filter is related to the use of a trellis-decoder in the HDTV receiver. In the Grand Alliance system, a four-state rate 2/3 trellis code is used in signal transmission. When a comb filter is not used in the HDTV receiver, a corresponding decoder comprises a four-state Viterbi decoder as discussed in "Channel Coding with Multilevel/Phase Signals", IEEE Transactions on Information Theory, vol. IT-28, no. 1, pp. 55-67, January 1982, by G. Ungerboeck.
On the other hand, when a comb filter is used for co-channel interference mitigation, the trellis decoding strategy now implements Viterbi decoding on an expanded trellis, the states of which correspond to the cascade of the states of the comb-filter and the trellis coder as is known in the art. That is, for a comb-filter with a delay of 12 symbols, the number of trellis states are extremely large. To simplify the Viterbi decoder, the Grand Alliance system converts the MPEG-coded and RS-coded and interleaved data-stream from serial to parallel, then uses 12 parallel trellis encoders followed by a parallel to serial converter at the transmitter. An optimal Viterbi decoder for use with a comb filter after the trellis encoder would require a 16-state decoder. This 16-state decoder implementation can be simplified to an 8-state decoder by using special properties associated with the comb filter resulting in a very small loss in performance, as will be explained herein below.
In contrast, when a variable comb filter is used, the optimal decoder is still a 16-state Viterbi decoder. However, simplification of the 16-state Viterbi decoder to an 8-state Viterbi decoder for use in conjunction with the variable comb filter is not possible without a significant loss in performance. Hence, for a variable comb filter, the trellis decoding strategy appears to have a much higher complexity.
Another problem associated with the variable comb filter is related to the synchronization symbols, and more specifically, the segment sync symbols introduced in the Grand Alliance transmission system. The segment sync symbols are used to derive accurate timing and framing information at the receiver. During a time in which the segment sync symbols are transmitted, the trellis-coders, whose outputs would correspond to the segment sync symbols, are frozen, i.e., no data is input to these trellis coders. After transmitting the segment symbols, the next symbol to be transmitted corresponds to the next encoder in sequence after the frozen trellis encoders. In other words, the segment sync symbols are not inserted into the trellis-coded stream, but rather replace the outputs of trellis encoders which correspond to the segment sync symbols.
For the receiver utilizing a comb filter, it is easy to show that the effect of the segment sync can be removed by adding two received data values corresponding to the symbols associated during and after transmission of the segment sync symbol. In contrast, however, this method results in a degradation in performance for the receiver utilizing a variable comb filter as the variable comb filter enhances the noise.